Septic shock, the leading cause of death in intensive care units, is characterized by vasodilation with decreased peripheral vascular resistance, which is often refractory to exogenously administered vasopressor agents. The most important determinant of peripheral vascular resistance is the tone of resistance arterioles, and modulation of tone in these arterioles results from a complex interplay of local vasodilators and vasoconstrictors. The mechanisms involved in the refractory vasodilation seen in sepsis have not been fully elucidated. The current proposal would be the first study to investigate microvascular abnormalities in a clinically relevant model of sepsis by testing responses of resistance arterioles to a range of endogenous vasoactive substances. The long-term objective of this project is to elucidate the pathophysiology of the abnormalities in vascular tone seen in patients with septic shock. The underlying hypothesis is that hypotension and abnormal distribution of blood flow in sepsis result from derangements in microvascular responses to endogenous vasoactive substances. The specific hypothesis is that responses of resistance arterioles in cremaster muscles of septic rats measured using in vivo videomicroscopy will differ from controls, and that elucidation of the mechanisms of differences in vasopressor responsiveness will aid in our understanding of important pathogenetic pathways and in the development of innovative therapies for septic shock. Specific aims: 1. To test the hypothesis that a general abnormality of microvascular reactivity is present in sepsis by comparing arteriolar responses to endogenous vasopressors in septic and control animals. 2. To evaluate potential effector mechanisms of sepsis-induced vascular hyporesponsiveness by measuring the effects of inhibitors of second messenger pathways. 3. To elucidate interactions between endogenous vasopressors and vasodilators in mediating vascular hyporesponsiveness in sepsis by testing the effects of nitric oxide synthase, cyclooxygenase, and lipoxgenase inhibitors on vasopressor-induced arteriolar constriction in septic animals. 4. To test the hypothesis that overproduction of nitric oxide by cytokine- inducible nitric oxide synthase plays a pivotal role in inducing vascular hyporesponsiveness in sepsis, first by comparing the effects of selective and nonselective nitric oxide synthase inhibitors on vasopressor-induced arteriolar constriction in septic animals, and then by measuring vascular responsiveness in transgenic septic animal deficient in inducible nitric oxide synthase.